This invention concerns a method of forming a liquid-tight closure over a container opening employing a non-adhesive stretch film, and the uses of such a closure in the laboratory.
A stretchable self-sealing plastic wrap has been used for many years in the chemical laboratory to provide a convenient water-tight closure on a test tube or other container. This stretch wrap known as PARAFILM.RTM. is manufactured by the American National Can Corporation and is described in the manufacturer's brochure titled "Parafilm M, the all purpose laboratory film and dispenser". The film, containing a substantial proportion of a cohesive paraffin wax, is permanently self-adherent when stretched over and around an object or a container opening. Irreversible elongation and thinning of PARAFILM.RTM. occurs during the stretch wrapping process. PARAFILM.RTM. is susceptible to most common organic solvents, and has a low softening temperature (approximately 60.degree. C.), well below that of boiling water.
Another adhesive wrap is described by Ketay et al., U.S. Pat. No. 2,500,549 in which a cellulose acetate film is partially coated on one side with a pressure-sensitive adhesive. The film is used to fit tightly over a container opening, attach to the container by the adhesive, and later be detachable from the container.
Non-adhesive stretch-wrap films which are composed of one or more thermoplastic layers and which include the polyethylene-based cling-type stretch wraps, are also known. Kishi et al., U.S. Pat. No. 4,073,782, describe a wrapping film containing either polyethylene or a copolymer of vinyl acetate and ethylene (or a mixture of these), plus sorbitan monooleate and liquid paraffin. The monooleate serves as an antihazing agent, while the viscosity and quantity of paraffin oil are selected to control the film cling.
Shadle, U.S. Pat. No. 4,082,877, describes a composite laminar film containing an elastomeric layer and a sealable layer of a polymeric composition containing an interpolymer of ethylene and alkenoic acid.
Climenhage et al., U.S. Pat. No. 4,337,188, describe a polyolefin composition for manufacture of a film having cling properties. The mixed composition includes a polyethylene blend, an elastomer copolymer including ethylene and propylene, and an agent selected from a class of organic compounds (certain organic liquids or waxy solids such as mineral oil and liquid polyolefin).
Cipriani et al., U.S. Pat. No. 4,379,197, describe a stretch wrapping film consisting of linear low density polyethylene (LLDPE) and sorbitan monooleate. The LLDPE is said to provide strength and toughness to the film while the monooleate provides the appropriate degree of cling.
Briggs et al., U.S. Pat. No. 4,399,180, describe a triple-layered coextruded polyolefin stretch wrap having a relatively thick core layer of linear low density polyethylene co-polymers, such as ethylene co-polymerized with at least one C.sub.4 to C.sub.10 alpha-olefin, and one or two relatively thin skin layers of highly branched low density polyethylene. The resulting multilaminate structure is said to have a reduced tendency to tear compared to other films.
Cooper, U.S. Pat. Nos. 4,425,268 and 4,456,788, describes a composition for stretch wrap film including a high molecular weight copolymer of ethylene and vinyl acetate, a linear copolymer of ethylene and higher alkene, and a tackifier to impart cling. Cooper also describes a multilayer composite stretch film composition including a first layer of a high molecular weight ethylene and vinyl acetate copolymer and a tackifier. A second layer which is adhered to the first, is principally linear low density polyethylene.
Breck, U.S. Pat. No. 4,657,982, describes polymer blends and films including up to 99% of a linear copolymer of ethylene and C.sub.4 to C.sub.8 .alpha.-olefin or a mixture of this linear copolymer and high density polyethylene or ethylene-vinyl acetate copolymer, and 0.5% -10% each of low molecular weight and higher molecular weight polybutene. The mixture of polybutenes is said to impart higher cling strength than that achieved with single polybutenes.
Knott, U.S. Pat. No. 4,671,987, describes a thin printable multilaminate stretch wrap which exhibits strong cling. The film includes at least two layers, one layer containing a blend of ethylene-vinyl acetate (EVA) copolymer and a tackifier, and a second layer free of tackifier (thereby allowing printing), including at least 50% linear very low density polyethylene (VLDPE). The film may also include a layer of LLDPE positioned between the tackified EVA layer and the VLDPE layer of the film.
Benoit, U.S. Pat. No. 4,833,017 describes a thermoplastic stretch wrap film fabricated from a polyolefin such as LLDPE in which one surface of the film possesses cling and the opposite surface, lacking cling, possesses a slide property obtained by bonding a particulate antiblock agent to the surface. The film is used for conventional unitized packaging of articles.
Japanese Patent 53,034,845, describes the use of chlorinated paraffin as a tackifier in an ethylene-vinyl acetate self-adhesive stretch film.